A pilot-scale engineered ecosystem (PSEE) to treat domestic wastewater was designed, built and successfully operated for 2.3 years. The system offers a low technology, low maintenance solution to the problem of environmentally sustainable on-site wastewater treatment. It uses no chemicals (unless phosphorus removal is required) and minimises energy and maintenance requirements through the use of sunlight and native biota.
The requirement for an environmentally sustainable method of wastewater treatment was highlighted by a water management survey of remote tourist resorts in Queensland and northern New South Wales. It was established that such a system should be commercially viable, legally compliant, easy to operate and maintain, and be positively integrated into the resort in terms of both aesthetics and acceptance.
Engineered ecosystems, which offer a more environmentally sustainable method of wastewater treatment, have been used in the Northern Hemisphere (Living Machines ©, Solar Aquatics ©) but have not been used in Australia to date. These systems were studied and modifications and adaptations made to allow for the Australian climate and the requirement to maximise environmental sustainability. The modifications and changes in design included:
• the removal of the greenhouse thus maximising the planted area without substantially increasing capital costs;
• the incorporation of dedicated algae troughs to increase the nutrient removal;
• the removal of the second stage of aerobic bacterial treatment to reduce energy input;
• the operation of the primary aerobic stage at a long sludge age in order to negate the need for frequent sludge removal and sludge treatment facilities; and
• the use of Australian native flora and fauna.
The operation and subsequent evaluation of the PSEE showed:
• compliance with the Australian and New Zealand standard for on-site domestic wastewater treatment in terms of BOD and TSS;
• a high degree of disinfection that can easily be increased through greater sunlight exposure to achieve compliance with the standard;
• removal of approximately one third of the influent nitrogen although a much higher removal may have been achieved if dissimilatory nitrate reduction to ammonia (DNRA) had not occurred within the PSEE;
• insignificant phosphorus removal;
• the ability to maintain treatment performance for between 3 to 6 days at double flow and load and the ability to regain performance 4 days after flows and loads have been returned to design levels;
• the ability to retain performance through periods of no flow;
• the ability to operate successfully at a very long sludge age (approximately 700 d);
• a reduction in nitrogen removal at the colder temperatures experienced during the winter months; and
• the native fauna that can be successfully used in such a system.
In terms of wastewater treatment performance and environmental sustainability, the PSEE was comparable or better than conventional on-site systems, the Northern Hemisphere engineered ecosystems and other alternative systems such as wetlands and aquaculture. Operation also showed a number of design modifications necessary to increase performance and maximise environmental sustainability:
• an upgrade of the anaerobic treatment stage: retention longer than 24 hours and the use of two chambers;
• intermediate sludge settlement and return to the anaerobic phase (for biannual removal) in order to prevent DNRA;
• partial aeration of planted tanks;
• the use of a fishpond at the end of the engineered ecosystem for final effluent polishing; and
• adoption of mosquito prevention techniques.
The use of algae troughs was not proven and hence careful consideration would be required before their inclusion in any subsequent engineered ecosystems.
If phosphorus removal is required, the system will need to include frequent sludge removal and/ or chemical addition. The removal of phosphorus by macrophyte and algal uptake was not sufficient to significantly lower the concentration.
However, the potential for engineered ecosystems for on-site wastewater treatment is high as they:
• can be used to treat combined wastewater (blackwater and greywater) or greywater;
• can be installed on a green field site or retrofitted to existing systems; and
• allow the wastewater to be used as a resource during treatment (e.g. macrophytes grown in the system can be used for landscaping, compost or fodder).
There is currently interest in the system not only from the tourism industry for which it was designed but also from developers of small residential areas and industries interested in using wastewater as a resource. It is hoped that the research will continue and the engineered ecosystem be trialled at a much larger scale (40 to 60 EP).